1. Field of the Invention
The present invention relates to a length measuring device.
2. Description of the Related Art
Such length measuring devices, of the kind described for instance in German Patent Disclosure DE 29 29 989 A1, serve to measure lengths as well as travel distances and are used in particular in processing machines for measuring the relative motion of a tool with respect to a workpiece to be machined, in coordinate measuring machines, and increasingly in the semiconductor industry as well.
A scale is used to measure the lengths and travel distances. The scale, is accommodated in a housing so as to be protected against environmental factors. Temperature changes inside the housing cause a change in length of the scale, depending on the material of the scale. These changes in length lead to measurement errors.
To keep such temperature changes as small as possible, it is proposed in German Patent Disclosure DE 101 64 404 A1 that the temperature of the interior of the encapsulated length measuring device be tempered in that there is a regulated flow of coolant through it.
The intent of this embodiment is to have the scale assume the temperature level of the machine tool. A disadvantage is that temperature changes that are generated by electrical components of the length measuring device itself are not taken into account.
The demands made of length measuring devices are becoming more and more stringent, and both higher resolution and greater precision and replicability of position measurement are constantly demanded. There should also be a compact mechanical construction as well as simple, malfunction-proof generation and transmission of measured values.
These demands require an encapsulated length measuring device with a scale accommodated in a protected fashion. The demand for high resolution dictates an ever smaller scanning spacing that is constant over the entire measured length. This is attained on the one hand by a relatively small scanning spacing of less than 100 μm between the scale and the scanning unit. On the other hand, high resolution is attained by guiding the scanning unit on the scale and/or on the housing in an unimpeded precise manner by having the mounting block coupled to the scanning unit via a coupling that is rigid only in the measuring direction. In all the other directions, this coupling enables a movement of the mounting block without affecting the precise guidance and movement of the scanning unit in the measuring direction.
A compact construction and malfunction-proof generation and transmission of measured values requires integrating an increasing number of electrical components in the scanning unit itself. Increasingly for this purpose, sensor chips, an array of scanning sensors, such as light-sensitive detectors if the scale is scannable photoelectrically, and signal processors, such as A/D converters, amplifiers, microprocessors, and interfaces are for example disposed in the least amount of space. The result is increased heat generation in the scanning unit, which heats the scale disproportionately because of the requisite small scanning spacing. This heating causes changes in length of the scale and measurement inaccuracies.